1. Field of the Invention
The present invention relates to apparatus for transforming a single laser beam into two parallel beams of adjustable spacing and intensity. The distance between the two output beams can be varied continuously over a wide range, and the distance between the output beams can be reduced to zero. Specifically, the invention relates to optical apparatus which is useful in a variety of optical measuring devices which perform rapid and accurate noncontacting measurement of object dimensions, hole dimensions, object positions, and sheet thickness.
2. The Prior Art
For the accurate measurement of the diameter, position, or thickness of soft, delicate, hot, or moving objects, noncontacting sensors must be used. Prior-art devices of this character include capacitive gauges, eddy-current gauges, air gauges, gamma- and x-ray gauges, and optical sensors. Only the optical and nuclear gauges can work at distances greater than a small fraction of an inch with sufficient sensitivity. The nuclear gauges permit large working distances; however, they are extremely expensive and susceptible to systematic errors due to slight variations in the chemical composition of the object being measured.
Optical sensors have advantages due to the nature of light itself. The principal advantages are:
1. They do not require direct mechanical contact between the sensor and the object to be measured.
2. The distance from the sensor to the object to be measured can be large.
3. The response time is limited to that of the photodetector and its electronics.
4. Light variations are directly convertible to electrical signals; and
5. The measurements are independent of the chemical composition of the object.
In such apparatus, it is desirable to produce two parallel light beams with a parallelism tolerance of several seconds of arc in the plane of the measurement.
For example, U. S. Pat. No. 2,812,685 discloses apparatus for the dimensional measurement of moving objects wherein two parallel light beams of variable spacing are used. There are two important disadvantages in the means disclosed for producing the two light beams. Firstly, the distance between the beams cannot be made less than the beam diameter, since the fixed halfsilvered mirror used to produce one beam begins to obscure the beam from the adjustable mirror used to produce the second beam for distances less than the beam diameter. Secondly, in order to preserve the parallelism of the two output beams, the angular alignment of the adjustable mirror must be maintained to very high degree when the beam spacing is varied. Furthermore, a slight tilt of either the adjustable mirror or the half-silvered mirror originating from any cause introduces adverse measurement errors. The tilts can arise from mechanical changes or from thermally induced mechanical changes.
While prior-art techniques for producing two parallel light beams of adjustable spacing are useful for some applications, they cannot be used for the accurate control of many industrial operations. For example, in the high-speed extrusion of aluminum rod, it is desirable to get readings with accuracies of 0.0001 inch. The extruded rod moves in every direction as it comes out of the die so that the sensor must be capable of fast, accurate measurements over a large measurement volume, i.e., several inches on a side, and with a short measurement duration. The measurement of sheet thickness in a rolling mill is an example where a high sensitivity sheet thickness sensor with a large working distance, large measurement range, and short measurement duration is required.